The Prince Rupert’s drop is one of science’s most longstanding “conundrums”: Tadpole-shaped glass drops so strong that they can withstand a .38 caliber bullet, but will explode into powder if the end is clipped.

And now, after nearly 400 years, scientists have finally figured out why.

In a new paper, a U.S., British and Estonian team determined that the drops get their peculiar strength by being a masterpiece of perfectly balanced forces.

Prince Rupert’s drops are manufactured by dropping a ball of molten glass into a beaker of room temperature water.

The exterior cools faster than the interior, leaving the whole drop enshrouded by a layer of steel-hard glass compressing inward.

This gives the drop its incredible strength, but also means that the core is composed of glass under immense tension.

This interplay is also why the tail is the drops’ Achilles heel.

By merely snapping off the thin end of the drop, it sends a chain reaction of tiny cracks shooting towards the stressed core — ending in a catastrophic breakdown.

“On one hand, the head can withstand hammering, and on the other hand, the tail can be broken with just the slightest finger pressure, and within a few microseconds the entire thing shatters into fine powder with an accompanying sharp popping noise,” contributor and Purdue University industrial engineering professsor Srinivasan Chandrasekar said in a statement.

The drops are named for Prince Rupert, a German prince who once lent his name to a vast swath of Canada. Rupert’s Land, which encompasses five modern-day Canadian provinces, was the colonial name given to all the land that drained into Hudson Bay.

In 1625, Rupert presented five of the drops to England’s Charles II — who immediately set his scientists to work on figuring out the secret on how an object could be so durable and fragile at the same time.

However, it took until the 21st century that researchers were able to crack the code of the Prince Rupert’s drop by immersing it in specially calibrated liquid and then shining an LED through the drop to illuminate its internal structure. That way, they were able to map where the drop was strongest.

In recent years, the incredible strength of the Prince Rupert’s drop has made it an internet favourite of sorts, as YouTubers subject the drop to increasingly creative torments, including gunfire and industrial presses.

The effort to unlock the mystery of the drops may seem like it’s merely a bunch of scientists playing with glass trinkets, but researchers noted that discovering the properties of super-durable glass could have untold benefits in an age of constantly shattering smartphone screens.